Currently almost all automobiles and light trucks have some version of a hydraulically controlled, hydraulic power steering system, which is susceptible to parasitic energy loss. In such hydraulic systems, a hydraulic pump is continuously engaged with the vehicle motor to provide a constant flow of hydraulic fluid, typically at a significantly high pressure, to the power steering system. The operator, however, does not always require power steering assist to steer the vehicle, and so there is a significant energy inefficiency that results in the waste of billions of gallons of gasoline in the world every year. What is needed is an improved, “on demand” power steering system, in which the hydraulic pump is intermittently engaged to the vehicle motor.
If the hydraulic pump of an “on demand” system is engaged with the vehicle motor every time the operator requires power assist, the hydraulic pump would be engaging and disengaging very frequently. Therefore, what is needed is a power steering system in which the hydraulic pump is intermittently engaged to the vehicle motor only when necessary to pressurize the system to a desired level to provide power steering assist.
In a conventional hydraulic power steering system, a fluid valve arrangement directs pressurized fluid to either side of a piston in a hydraulic cylinder coupled to the steering mechanism to provide steering assist. When the operator turns the steering wheel, the system pressurizes one side of the piston, and fluidly connects the other side to a fluid reservoir, which is at a relatively low, atmospheric pressure. When the operator centers the wheel, each side of the piston fluidly connects to the reservoir. For some driving conditions such as when driving over pot holes and bumps, the driver may experience some driving instability in the center position since the steering system provides minimal damping to the hydraulic piston.
What is needed is an improved steering system that fluidly connects both sides of the piston in the hydraulic cylinder to a fluid to provide damping for the center position of the steering mechanism. This damping would also mitigate driving instability due to cross-steering linkage, tire vibration induced, suspension resonances. In addition, fluidly connecting both sides of the piston to each other rather than the reservoir in the center condition helps to minimize parasitic, fluid flow losses in the steering system. Therefore, what is yet further needed is an improved hydraulic power steering system that not only provides hydraulic cylinder damping during the center condition, but that also minimizes parasitic losses by fluidly connecting each side of the piston of the hydraulic cylinder.
Conventional hydraulic power steering systems supply the hydraulic cylinder with fluid pressurized to a high level that is suitable for driving situations requiring maximal steering assist. In many situations, however, the operator may only need a small or intermediate amount of steering assist, and it is not necessary to supply the hydraulic cylinder with highly pressurized fluid. What is still yet further needed, therefore, is an improved hydraulic power steering system, that provides low and/or intermediate pressures of fluid to the hydraulic cylinder.